Cumulative Monthly Totals

[cbetts75]

Hi,

I'm trying to create an expression that will calculate cumulative monthly totals but my expression seems to only calculate totals for all months
e.g
Month Direct Despatches Cum Direct Despatches
2006/03 1580 21867
2006/04 10681 21867
2006/05 9606 21867

The expression I’m using is
Cum Direct Despatches: (Select Sum([Direct Despatches]) from QRY_DirectDespatches_ByMonth_ByModel)

The query should display the following results -
Month Direct Despatches Cum Direct Despatches
2006/03 1580 1580
2006/04 10681 12261
2006/05 9606 21867

I'm using Access 2002 on XP.
Can anyone please advise me where i'm going wrong?!
Thanks in advance for the response